Powder coatings are widely used to provide a decorative and/or protective coating on substrates. They are becoming increasingly popular because they are applied in a solid state or slurry. Unlike conventional liquid coating compositions, the powder coating compositions use little or no solvents. In addition, solid state application permits the powder to be collected, purified and re-used.
UV curable powders have been used in powder coating compositions. Typically, UV curable powders are formulated from solid crosslinkable base resins with low Tg, such as crosslinkable polyesters; crosslinkable co-polymerizable crosslinker resins, such as vinyl ethers; photoinitiators; flow and leveling agents; and, if necessary, pigments and fillers.
During coating operations, UV curable powders are applied to a substrate in the usual fashion, using electrostatic spray techniques. The coated substrate is then heated for as long as it takes to drive out substrate volatiles and fuse the powders into a smooth molten film. Immediately following fusion, the molten film is exposed to UV light, which, in an instant, cures and hardens the film into a durable, smooth, attractive coating.
In certain applications, it is necessary or desirable for the powder coating to have a surface that is preferably smooth in appearance, but has a low gloss or shine. Such applications are those where low gloss is aesthetically desired, or where glare from the coating surface can interfere with the safe or proper use of the coated article, such as desks, tables, counter tops or other horizontal work surfaces, optical devices, motor vehicles, aircraft and ships.
One drawback of UV curable powders is that it is very hard to produce a low gloss (i.e., matte) coating from such powders since the coatings resulting therefrom tend to be glossy. Gloss reduction can normally be obtained in traditional powder coatings through the introduction of matting agents, such as fillers or waxes, which rise to the surface during curing and cause matting through disruption of the surface of the coating. However, due to the fast cure rate of UV curable powders, conventional fillers or waxes cannot flocculate to the surface fast enough to produce the low gloss and they remain trapped within the coating. Higher amounts of fillers or waxes, which can be employed to overcome this problem, tend to cause the powders to block or cake during normal storage and/or produce coatings with severe orange peel, limiting the amount of gloss reduction that could be attained.
One approach to tackle the foregoing problem associated with obtaining low gloss coating's from UV curable powder coating compositions was presented in U.S. Pat. No. 6,017,593. In U.S. Pat. No. 6,017,593, low gloss was obtained by the use of a mixture of crystalline and amorphous resins and by adding a cooling step after the melting step but before, the photoinitiated curing step during which the crystalline resin recrystallizes. However, the process in U.S. Pat. No. 6,017,593 is limited in practical applications to only those substrates having a uniform heating and cooling profile. Thus, the process of U.S. Pat. No. 6,017,593 gives rise to differential crystallization, differing gloss, and a mottled appearance when used with substrates having variable heating and cooling profiles, such as non-metallic substrates or those having varying thicknesses, sharp edges and corners.
Therefore, it would be desirable to provide a method for producing coatings with a low gloss appearance from UV curable powder coating compositions on substrates having variable heating and cooling profiles, such as non-metallic substrates.